Conventional hearing aids rely on amplification of sound to improve hearing. This approach has several disadvantages. First, acoustic energy applied to the ear canal results in the occlusion effect, which occurs when bone-conducted sound energy trapped within the ear canal vibrates the cartilaginous portion, and results in unnatural sound quality due to an increased low frequency gain. This unnatural sound quality is especially bothersome to people with mild hearing loss. The occlusion effect increases with the volume of trapped air within the ear canal. Second, the output sound energy from the speaker may escape and re-enter the microphone, causing feedback when the amplification from microphone to speaker is greater than the attenuation from speaker to microphone. The problem of feedback is particularly problematic in patients with moderate or severe hearing loss where significant amplification is required, especially in the high-frequency region. It is also a problem for miniaturized devices where the microphone and acoustic driver are close together.
Some hearing aids attempt to solve the occlusion effect by adding a vent to the earmold to allow sounds trapped in the ear canal to escape. A larger vent diameter and shorter vent length would be more effective in reducing occlusion. However, a tradeoff of a larger vent diameter and short vent length is that such a vent provides less attenuation from a speaker to a microphone and thus, increases the likelihood of feedback. The problem feedback is overcome by increasing the separation between the microphone and speaker, usually by increasing the size of the hearing aid (in order of increasing size and visibility) from completely-in-the-canal to in-the-canal to in-the-ear to behind-the-ear. Patients, however, generally do not want to wear larger hearing aids due to their appearance and attached stigma. Although digital feedback management techniques can be applied, the state-of-the-art feedback management algorithms lead to signal degradation.
Micro hearing aids have been developed, but they suffer from the feedback problem just described. One of the newer hearing aids on the market (Lyric, InSound Medical Inc.) is small enough to be inserted deep into the bony part of the ear canal without being visible. The device eliminates the stigma attached to hearing aids and reduces the occlusion effect by reducing the amount of sound generated in the ear canal. However, due to feedback problems associated with a short distance between microphone and speaker, the micro hearing aids are typically only suitable for persons with mild hearing loss who do not require high amplification.
Alternatives to conventional hearing aids include the semi-implantable, implantable or fully implantable middle ear transducer. An early device (Direct System, Soundtec Inc.), now withdrawn from the market, consisted of a magnet attached to the ossicles (incudostapedial joint). The magnet was driven by an electromagnetic field produced by the external unit, consisting of a deeply fitted earmold housing an inductive coil, held approximately 2 mm lateral to the tympanic membrane, and a behind the ear (BTE) device housing the other electronic parts. The Vibrant Soundbridge (Med-El Corp.) consists of two parts attached by magnets—an implanted part consisting of the receiving coil, electronics and transducer, and an external part housing the microphone, speech processor, battery and transmitting coil. The Carina (Otologics LLC), which is available in Europe and currently under clinical trial in the US, is fully implantable. These devices translate sound energy into mechanical energy via a piezoelectric actuator that directly drives the ossicular chain. By having a mechanical rather than an acoustic output, the problem of acoustic feedback is eliminated. By driving the ossicles directly, the device may eliminate the occlusion effect and can provide a better sound quality compared to conventional hearing aids. However, major disadvantages of these devices that have reduced their acceptance include prohibitive cost, the need for an invasive surgery, the need for a second device with a microphone, and the requirement of an additional surgery for removal if there is a problem with the device. On the positive side, clinical studies showed that most patients preferred the sound quality of their middle ear implant over their hearing aid and thought that the feedback problem had been resolved.
The current state of art does not provide a satisfactory way to restore hearing without one or more of the follow disadvantages; feedback, occlusion effects, easily visible, stigma, invasive surgery, expensive and/or surgery for removal. Thus, there exists a desire for a device which overcomes one or more of the aforementioned drawbacks.